I have a User who received 3 emails which he tried to move to a subfolder via drag'n'drop. After moving them he viewed the folder but it was shown as empty.

This is a simplified version of the folder structure:

Inbox
+-Monthly Reports
+-2011
+-2012
+-FooBar

The emails were originally moved to the FooBar folder. Viewing this folder shows it as empty. If I issue a "All Subfolders" search on the Inbox or "Monthly Reports" level I get the invisible emails as a search result. Issuing the same search on the 2012 or FooBar folder yields a result WITHOUT the mysterious Emails.

I tried to select the emails in the search result and move them to a different folder, but the emails remain invisible and the above situation remains the same.

I tried to select the emails in the search result and move them to a different folder, but the emails remain invisible and the above situation remains the same.

This part of the whole problem had me confused. It turns out that moving those emails into that temp folder actually worked (they showed up inside of it), but they also still showed up on those searches on the Monthly Reports folder. It wasn't until I completely moved the whole temp folder into a different mailbox (PST file this time), that they stopped showing up in those searches. Which means that even though the temp folder wasn't a subfolder of Monthly Reports it's contents showed up in a "All subfolders" search ON Monthly Reports.

I moved the emails back into the FooBar folder and they display as they should. Mystery solved, at least for me.